This document discusses the helical antenna. It provides information on the radiation pattern, polarization, directivity, beamwidth, input impedance, and examples of calculations for various helical antenna parameters. The helical antenna has an omnidirectional radiation pattern perpendicular to its axis, with linear polarization parallel to the axis. It can achieve higher directivity than a dipole and is used for applications such as satellite and radiometry systems due to its broadband and circularly polarized radiation. However, it requires more space than a dipole due to its larger size.

1. Prof. T. S. Patel

2. Outline:
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7. • The antenna acts similarly to an
electrically short dipole
ormonopole, and the radiation
pattern, similar to these antennas is
omnidirectional, with maximum
radiation atright angles to the helix
axis.
• The radiation is linearl! polari"ed
parallel to the helix axis.

9. Parameter:

10. • HPBW: • FNBW:

11. • Directivity:
The gain of the helix
antenna can be
approximated by:
𝐷0 =15(C/λ)2N(S/λ)
• Gain:
The gain of the helix
antenna can be
approximated by:
• G= 𝐷0 =15(C/λ)2N(S/λ)
(lossless helix)

12. • Axial Ratio: • input Impedence:

13. • Normalized far field
pattern:
𝐸 = 𝑠𝑖𝑛(
𝜋
2𝑁
)𝑐𝑜𝑠𝜃
𝑠𝑖𝑛(
𝑁𝜑
2
)
𝑠𝑖𝑛(
𝜑
2
)
where, 𝜑 = 𝛽(𝑆𝑐𝑜𝑠𝜃 −
𝐿𝜃
𝜌
)
• C=λ, α=14◦

14. Example: A 16 turn helical beam antenna has circumference
of λ and turns spacing of λ/4 find: 1. HPBW 2. Axial ratio 3. Power
pattern
Ans:
N=16
C=λ
S=λ/4
HPBW=26◦
AR=1.03≈1
G=60(17.78 dB)

15. Example: Calculate the directivity HPBW and terminating
resistance for helix operating in axial mode
Input:
N=9
f=2 GHz
C=0.8 λ
S=0.2 λ
Ans:
D0 = 17.28
HPBW=48.45◦
Terminating Resistance
RL=Rin=140(C/λ)=112 Ω

16. Example: A helix operated at 2 GHz has dimensions S=5 cm,
D=10 cm, N=20, find out 3 dB beamwidth and directivity.
Input:
• f= 2 GHz
• S=5 cm
• D=10 cm
• N=20
Ans:
• λ=c/f=15 cm
• C=πD=31.42 cm
• HPBW=9.61◦
• D0 = 438.8 (26.42 dB)

17. • It is used for transmission and reception of VHF signals through ionosphere
• It is used for satellite and radiometry applications.
• Advantages:
 It is simple in design.
 it uses circular polarised pattern,
 It can be used for broadband
applications due to wider
bandwidth.
 It can be used at HF/VHF
frequencies for transmission and
reception.
 It offers higher directivity.
 It is very robust in construction.
• Disadvantages
 It is large in size. This requires
more space for installation.
 For higher number of turns its
efficiency decreases. The maximum
efficiency of about 80% can be
achieved with the use of 3 to 4
turns.
 It is higher in cost.